Term U-shaped cost curves Definition: The family of short-run cost curves consisting of average total cost, average variable cost, and marginal cost, all of which have U-shapes. They are U-shaped because each has high but falling cost at low quantities of output, which then reaches a minimum, then has rising cost at larger quantities of output. Although the average fixed cost curve is not U-shaped, it's occasionally included with the other three just for sake of completeness.